The Regimental or Battalion Aid Station were small mobile installations designed to give ONLY such emergency medical treatment as was necessary to enable patients/casualties to be evacuated further to the rear .The main treatment consisted of alleviation of pain, application of dressings, splints, or plasma. No surgery was performed. Casualties were placed on litters, made comfortable by the use of blankets and administration of fluids or food. The treatment was short and intended to avoid any delay in the evacuation process.

This is from a photo series at an aid station in Italy in 1945. One shot shows stacks of litters for the litter teams to grab (assuming they left the patient on the litter they used coming in), as well as a small pile of crates & cartons for the medics to refill their bags - mostly Carlisle bandages, etc. there was a stack of blankets available, too. In this series of photos, the Aid Station and the Clearing Station were co-located - casualties were brought in, stabilized, and moved by ambulance to a field hospital about 5 miles back.

A better example is seen here - there are about 30 shots taken if you can find the full set.